Contour Lines

Contour lines are the means of relating the vertical dimension
(the third dimension) of the topography of an area to the
two-dimensional surface of a topographic map. Contour lines
should be visualized as the intersection of the land surface with
a series of equally spaced, horizontal planes that pass through
this surface. The vertical distance separating these planes is
termed the contour interval (C.I.). Contour intervals are
determined by the map scale and the amount of topographic
variation within the map area. As viewed on a topographic map,
the spacing of contour lines (but not the contour interval)
varies according to the changes in slope angle of topographic
features (Figure 7.5). Because of this
relationship, a few simple Rules of Contour Lines will prove
helpful in interpreting the vertical dimension of topographic
maps.

Gentle slopes (low angle from horizontal) on
topographic maps with a given interval will be
represented by widely spaced contour lines.

Steep slopes (high angles from the horizontal) on
topographic maps with a given contour interval will
be represented by closely spaced contour lines. Thus,
a vertical cliff large enough to be represented as a
topographic feature would be represented by contour
lines that merge (i.e., stacked together because of
the 90° slope).

Contour lines that cross streams flowing through
valleys of noticeable relief will form a V-shaped
deflection with the apex of the V pointing upstream (Figure 7.6). This relationship
between contour lines and stream valleys is referred
to as the RULE of V's. Because a V will always point
upstream, the orientation of V's can always be used
to determine the direction of the slope of the
surface as well as the direction of the flow of the
water.